According to a senior U.S. defense official, the FBI has uncovered between 20,000 and 30,000 pages of “potentially inappropriate” emails between Allen and Jill Kelley, a 37-year-old Tampa woman whose close friendship with Petraeus ultimately led to his downfall.

The "voluminous collection of emails" between the pair occurred between 2010 and this year.

Like Petraeus, Allen is a four-star general with a storied career in the armed forces. A deputy commander during the war in Iraq, Allen took over Petraeus' role as commander of U.S. operations in Afghanistan in 2011, when the latter was appointed to head the CIA.

He has been appointed to take over NATO command in Europe next year, pending Senate confirmation. His nomination has been put on hold, according to Pentagon chief Leon Panetta. Allen will remain in command of NATO forces in Afghanistan.

Kelley, the daughter of Lebanese immigrants who moved to the Philadelphia in the 1970s, is married to Dr. Scott Kelley, a surgeon formerly of the Moffitt Cancer Center. They live in a nearly 5,000-square-foot colonial-style home that they bought for about $1.5 million, according to property records.

Reports from the Tampa Bay Times and the AP describe Kelley as an active member of the Tampa social scene whose mansion is the "place to be seen" for high-ranking officers at U.S. Central Command, which Petraeus ran before leaving to run the war in Afghanistan.

The Times reports that in the decade since the Kelleys moved to Tampa, one or both of have been the subject of lawsuits "seeking payment of real estate and credit card debts intermingling with catered parties and A-list guests as the couple sought to establish themselves in Tampa."

On Sunday, the Federal Bureau of Investigation referred to the Department of Defense a matter involving General John Allen, Commander of the International Security Assistance Force (ISAF) in Afghanistan.

Today, I directed that the matter be referred to the Inspector General of the Department of Defense for investigation, and it is now in the hands of the Inspector General. I have informed the Chairman and Ranking Member of the Senate Armed Services Committee. The House Armed Services Committee has also been notified.

While the matter is under investigation and before the facts are determined, General Allen will remain Commander of ISAF. His leadership has been instrumental in achieving the significant progress that ISAF, working alongside our Afghan partners, has made in bringing greater security to the Afghan people and in ensuring that Afghanistan never again becomes a safe haven for terrorists. He is entitled to due process in this matter.

In the meantime, I have asked the President - and the President has agreed - to put his nomination on hold until the relevant facts are determined. I have asked both Senators Levin and McCain that the confirmation hearing on General Allen's pending nomination to be Commander of United States European Command and Supreme Allied Commander, Europe be delayed.

The President has nominated General Joseph Dunford, Assistant Commandant of the Marine Corps, to succeed General Allen at ISAF. I respectfully requested that the Senate act promptly on that nomination.

*This story originally stated that Dr. Kelley is currently a surgeon at the Moffitt Cancer Center. According to the Moffitt Cancer Center, Dr. Kelley has not worked there since 2008. We regret the error.